The object of the invention is a method for treating plastic film in packaging by wrapping, according to which method plastic film web used as packaging material is wrapped around the object to be packaged, the said web being treated thermally to seam the plastic films and/or to cut off the plastic film web. In the packaging sector, a widely used packaging method is one where the object to be packaged is wrapped inside a thin plastic film. The plastic film used in such a case is usually plastic film web which is unwound from a roll and wrapped around the object to be packaged.
The object of the invention also includes a device in a plastic film wrapping machine, which comprises wrapping means for wrapping the plastic film around the object to be packaged, and treatment means for thermally treating the plastic film, the said treatment means comprising a seaming device for seaming the plastic films and/or a cutting device for cutting off the plastic film web. Thus the method and device are applicable to a wide range of packaging machines. There are manual, semi-automatic and fully automatic wrapping machines.
Manual wrapping machines are usually turntable machines in which only the object to be packaged is rotated by means of a motor. Other functions, such as moving the object, start of wrapping, lifting and lowering the plastic film roll carriage, seaming, cutting and finishing of wrapping are all carried out manually. There are also manual rotary arm wrapping machines, in which the plastic film roll is rotated around the object to be packaged by means of an arm.
Semi-automatic packaging machines are usually also turntable machines or rotary arm wrapping machines. In these machines, the object to be packaged has to be moved to the wrapping station and away from it separately. These machines also lack automatic plastic film seaming or automatic plastic film web cutting.
In a fully automatic packaging machine, the object to be packaged is moved and the package handled automatically. The pallet on which the object to be packaged Is placed is fed into the wrapping machine by means of a conveyor, the object is wrapped in plastic film, the plastic film is seamed, the plastic film is cut off and the packaged object is conveyed away from the wrapping machine. All this takes place automatically. The handling of the next object to be packaged also starts automatically, in the same manner. With a fully automatic packaging machine, human intervention is usually only needed when the roll of plastic packaging film runs out. Someone is then needed to change the empty roll for a full one. Fully automatic plastic film wrapping machines are usually ring wrapping machines, turntable machines or rotary arm wrapping machines. Ring wrapping machines are generally always fully automatic.
The method and device relating to the invention can be applied to any of the above mentioned packaging machine types. The greatest advantages are, however, achieved with a fully automatic packaging machine, the efficiency of its operation increasing substantially as a result of the method and device relating to the invention. This is why the invention is described in the following, by way of an example, when connected to an automatic ring wrapping machine. It is, however, obvious that this only serves as an example, which does not restrict the use of the invention in other packaging machines.
A known plastic film packaging machine is the type of ring wrapping machine in which a roll containing plastic packaging film is placed in a wrapping carriage positioned in a circular rotation ring. The object to be packaged is placed in the centre of the circular rotation ring. Packaging takes place so that the rotation ring with its wrapping carriage rotates around the object to be wrapped, thus unwinding the plastic packaging film from the roll and winding it around the object to be packaged. Alternatively, the object to be packaged may rotate and the plastic film web be unwound from a plastic film roll moving vertically, which is the case with turntable wrapping machines.
On a known ring wrapping machine, when packaging is started, the end of the plastic film web unwinding from the roll is fastened to the grip of the gripping unit incorporated in the packaging machine. By means of the gripping means, the leading end of the plastic film web is held in place when wrapping is started. In this machine, the gripping means is positioned on the frame of the packaging machine in such a way that it will not rotate with the rotation ring. When at least one round of wrapping film web has been wrapped around the object to be packaged, the end of the plastic film web is detached from the hold of the grip. The detached end of the plastic film web then falls on top of the already wound layer of plastic film web and remains under the next plastic film layers which are wound over it. Since the surface of the plastic film web is sufficiently tacky, the layers of the package which are on top of each other will not slide but remain firmly in place.
In a ring wrapping machine, packaging is stopped when a sufficient amount of plastic film web has been wound around the object to be packaged, by seaming the plastic film web and cutting its end off. When seaming takes place, at least the two topmost web layers are seamed together and the web is cut adjacent to the seam. Seaming is usually done so that once the actual wrapping has been completed, the seaming counterpiece of the seaming device is placed above the plastic films of the package, at a distance from the topmost plastic film. After this, another one or two layers of plastic film are wrapped around the package, thus leaving the seaming counterpiece under these layers. The actual seaming takes place by bringing several band-like thermal resistors heated to a suitable temperature to the position of the seaming counterpiece, and pressing the said resistors momentarily against the seaming counterpiece. As a result, the seaming layers of the plastic film web are pressed between the seaming counterpiece and the thermal resistors, which causes the plastic films of the seaming layers to melt and adhere to each other at the points where the thermal resistors are located.
In a known ring wrapping machine, the thermal resistor bands used in seaming are usually parallel with the plastic film web, thus forming several adjacent seams having the same length as the resistor bands. The thermal resistor bands are parallel with the plastic film web because cross-web thermal resistors might melt and cut the plastic film web when seaming takes place. When the thermal resistor is moved away from the vicinity of the seam, the seam cools rapidly and the seaming counterpiece can be pulled away from between the packaging plastic layers.
Before the plastic film web is cut off, the gripping means of the gripping unit incorporated in the machine grips the plastic film web in the vicinity of the cutting-off point so that the end unwinding from the roll, which remains free, can be taken close to the next object to be packaged in order to start its packaging. The plastic film web is cut off adjacent to the seam by bringing a thermal resistor wire close to the plastic film web running from the package to the roll. When the thermal resistor wire is heated, the plastic web also heats up to such an extent that it is cut off by melting. After this, the wrapping plastic film package is complete.
Once the packaged object has been removed from the packaging machine and a new object has been brought in its place, the packaging of the next object is started by bringing the end of the plastic film web, by means of the grip, close to the object to be packaged and proceeding with the wrapping as described above.
The aim of the present invention is to increase the efficiency of operation of the plastic film packaging machine disclosed above and to achieve a new method and device for finishing off the plastic packaging film once the desired number of plastic film web layers have been wrapped around the object to be packaged.
It is characteristic of the method relating to the invention that in packaging by wrapping, plastic film is treated, so that one or more plastic film webs are heated by means of at least one heat radiator in order to seam the plastic film webs together and/or to cut off the plastic film web.
According to an advantageous embodiment, two or more plastic film webs are seamed together in such a way
that a heat-absorbent counterpiece is placed under the plastic film layers at the seaming point, the said counterpiece being heated by means of an infrared heat radiator through the plastic film layers, and
that the counterpiece is allowed to absorb the heat from the infrared heat radiator so that it heats up and at the same time heats the plastic film layers to be seamed in such a way that the plastic film layers are seamed together.
According to another advantageous embodiment, the plastic film web (22) is cut off in such a way
that a heat-absorbent counterpiece is placed behind the plastic film web at the cutting-off point, the said counterpiece being heated by means of an infrared heat radiator through the plastic film web, and
that the counterpiece is allowed to absorb the heat from the infrared heat radiator so that it heats up and at the same time heats the plastic film web to be cut off so that the plastic film web is cut off.
It is characteristic of the device relating to the invention that the plastic film wrapping machine comprises at least one heat radiator by means of which one or more plastic film webs can be heated in order to seam them together and/or the plastic film wrapping machine comprises at least one heat radiator by means of which the plastic film web can be heated in order to cut it off.
According to an advantageous embodiment of the device relating to the invention, the seaming device comprises an infrared heat radiator and at least one heat-absorbent counterpiece, and means for placing the infrared heat radiator and the heat-absorbent counterpiece on opposite sides of the plastic film layers to be seamed.
According to another advantageous embodiment of the device, the cutting device comprises an infrared heat radiator and at least one heat-absorbent counterpiece, and means for placing the infrared heat radiator and the heat-absorbent counterpiece on opposite sides of the plastic film layers to be cut off.
The invention enables both more rapid seaming and cutting and at the same time improved reliability and appearance of the seam. According to yet another advantageous embodiment, the infrared heat radiator is placed inside a tubular gripping counterpiece, the said gripping counterpiece having slots or apertures through which the heat radiation is able to pass to the counterpieces.